As antennas used in a wide band communication system, a multiple element antenna, a spiral antenna, a log periodic antenna, and the like are known.
The multiple element antenna is an antenna configured to obtain wide-band antenna characteristics by combining many antenna elements each having slightly different frequency bands. This multiple element antenna has a superior characteristic as to wide-band property, but there is a need to combine multiple antenna elements, which leads to a difficulty to adjust a feeding impedance of each antenna element and adjust a resonance frequency thereof. The spiral antenna and the log periodic antenna are simple in structure but their overall volumes are large and, in addition, their directional characteristics are limited only to a ground plane and a vertical direction when attached with the ground.
Also, in general, widening of practicable frequency bands of the multiple element antenna, the spiral antenna, and the log periodic antenna makes designing and adjustment thereof very difficult. Therefore, it has conventionally been difficult to realize a wide band antenna whose mass production is easy.
In recent years, a wide band communication system, such as a UWB system, has been applied in various fields. Also in an automobile, a mobile terminal, such as an on-vehicle wireless device, a portable telephone, or a PDA (Personal Digital Assistance) , a radio wave sensor, or the like is used. For instance, it is becoming not rare that an AM/FM radio receiver, an on-vehicle TV set, a GPS receiver, a satellite digital broadcasting receiver, a cellular phone, an ETC unit, a Bluetooth device, and a W-LAN are used in one automobile.
When terminals or systems that use frequencies in various bands are used like in this case, there arises a necessity to, for instance, attach many antennas to one automobile, which leads to a problem that an antenna installation space is increased as well as a problem that a cost is unusually increased.
One object of the present invention is to provide a low-cost wide band antenna having an ultra-wide band and high performance, with which it becomes possible to solve all of the problems described above.